The invention relates generally to control circuits and, more particularly, to control circuits for use in providing power to an electrical system.
There are many electrical systems that require power to be applied to various subsystems and/or components within the system in a specific time sequence during system power up. For example, many computer systems apply supply voltages to different subsystems on the computer motherboard in a predetermined time sequence when the computer system is turned on. In many cases, the delay period between the application of two successive voltages in such a system is critical to the proper operation of the system. For this reason, voltage sequencing circuits have been developed to appropriately and reliably sequence the application of voltages in such systems. These voltage sequencing circuits are typically implemented on a voltage controller chip housed within an integrated circuit (IC) package. The IC package is mounted on a circuit board within the electrical system where it is coupled to one or more power supplies. When the electrical system is activated, the voltage controller chip causes appropriate voltages to be applied to the various subsystems and components within the electrical system in a predetermined sequence.
Some voltage sequencing circuit designs of the past have been relatively complex. For example, in one prior voltage sequencing circuit, an oscillator and a digital counter are used to provide proper timing within the circuit. After a first voltage is applied in the system, the digital counter begins to count the number of signal cycles being generated by the oscillator. When the count value reaches a predetermined number, the voltage sequencing circuit causes the next voltage in the sequence to be applied. If there are further voltages in the sequence, an additional oscillator and counter is needed for each voltage. As can be appreciated, voltage sequencing circuits of this type are complex, require a significant amount of die area to implement, and are typically expensive. As is well known, it is generally desirable to minimize component costs within a system, particularly in systems that will be manufactured in large quantities. It is also generally desirable to minimize the size of the individual components within a system.
In another previous design, a voltage sequencing circuit is implemented within an IC package having a dedicated output pin for connection to an external capacitor. After a first voltage is applied in the system, a current source within the voltage sequencing circuit begins to charge the external capacitor. After the voltage on the external capacitor reaches a predetermined value, a drive unit within the voltage sequencing circuit signals an external switch, via another pin of the IC package, to apply the next voltage in the sequence. Thus, the second voltage is applied a predetermined time period after the first voltage, with the predetermined time period being related to the charging time constant of the external capacitor. If there are further voltages in the voltage sequence, an additional output pin and external capacitor is needed for each voltage. As is well known, because IC packages are typically pin constrained, it is generally desirable that the number of terminals that are communicating with an exterior environment to perform a particular function be minimized. In addition, by minimizing the number of pins used by an IC, a smaller package can often be used which will take up less space on the surface of a circuit board. It is also generally desirable that the number of external components that need to coupled to an IC to enable the internal circuitry to operate be minimized.